Catalyst compositions based on well defined donor ligand containing metal complexes, referred to as post-metallocene complexes have been shown to give products having better comonomer incorporation and narrow molecular weight distribution. However, these catalysts often have poor high temperature stability and suffer from poor catalytic efficiencies, especially at elevated polymerization temperatures.
Examples of one type of the foregoing post metallocene catalysts are disclosed in U.S. Pat. No. 6,827,976, where Group 3-6 or Lanthanide metal complexes, preferably Group 4 metal complexes, of bridged divalent aromatic ligands containing a divalent Lewis base chelating group are disclosed.
Higher solution reaction temperatures are particularly desired for olefin polymerizations in order to improve operating efficiency and to produce long chain branching in the resulting polymer. Long chain branching in olefin polymers is believed to result in one embodiment from the incorporation of vinyl terminated polymers, generated in situ by β-hydride elimination that results in vinyl group formation in growing polymer chains. These processes are benefited by use of high reaction temperatures and high monomer conversion conditions. Accordingly, selection of catalyst compositions capable of incorporating long chain branching, such as for example reincorporation of in situ produced vinyl terminated polymer, under the foregoing extreme reaction conditions is highly desired.
We have now discovered that certain metal complexes may be employed in a solution polymerization process to prepare high molecular weight ethylene containing interpolymers containing relatively large quantities of long chain branching therein at high olefin conversions if certain process conditions are observed. The resulting polymer products possess desirable properties such as better flexibility, reduced density (greater comonomer incorporation) and improved processability (less energy required for extrusion, reduced melt fracture and reduction of surface imperfections or “sharkskin” formation). In addition, we have discovered that these catalyst compositions retain their high catalyst activity and long chain branch forming ability using relatively low molar ratios of conventional alumoxane cocatalysts. The use of reduced quantities of alumoxane cocatalysts (reduced by up to 90 percent or more, compared to the quantities employed in conventional processes) allows for the preparation of polymer products having reduced metal content and consequently increased clarity, improved dielectric properties and other enhanced physical properties. In addition, the use of reduced quantities of alumoxane cocatalysts results in reduction in polymer production costs.